This invention relates to protective coatings and to fasteners coated by them, and more particularly to such coatings and fasteners capable of protecting one or both of two dissimilar metals assembled together, from structural corrosion or deterioration.
The invention is applicable to use with a number of different metals and combinations of metals. It is especially applicable to the coating of titanium. A particular application relates to titanium fasteners commonly used in the aluminum structures of aircraft and the like.
It is common practice to assemble aluminum or aluminum alloy structures, such as those of aircraft, with high strength fasteners of titanium or titanium alloys. It is well-known that galvanic action due to electro-chemical coupling effects present in such assemblies often results in undesirable corrosion of the aluminum or titanium elements, or both. It is known that aluminum has a tendency for galvanically induced corrosive attack in contact with titanium, if wet. Furthermore, the corrosion susceptibility of these structures is increased by harsh saline or acidic environments frequently encountered. When the fasteners are of the interference-fit type such as commonly used in the aircraft industry, the problem is further compounded by the fact that a coating on a fastener must be tough and adherent enough to withstand the force fitting operation. Such coatings must also be held to close tolerances.
A number of expedients have heretofore been proposed to reduce or eliminate such galvanic corrosion, among which have been: plating the fasteners with cadmium or aluminum; substitution of steel for titanium fasteners; coating the fasteners with organic or inorganic coatings; use of wet primers or elastomeric sealants during installation; and coating the fasteners or structural exteriors with paint such as a zinc chromate type. Chemicals including phosphates, molybdates, and silicates of some metals, such as sodium silicate, and zinc salts including zinc molybdate, zinc phosphate, and zinc oxide have also been found to be effective as corrosion inhibitors. Such materials are believed to prevent corrosion by a variety of mechanisms, such as forming an electrically non-conductive molecular layer on the metallic substrate, decreasing the permeability of the coating, forming a chemically resistant compound on the metallic substrate, or making the coating material hydrophobic to thus prevent water-borne corrosive materials to reach the substrate, for example.
The several types of coatings and corrosion inhibitors heretofore used have presented problems such as failing to give complete protection, inadequate toughness or adherence, and excessive expense. Even those most widely used in the aircraft industry, namely cadmium plating, organic and inorganic coatings, and sealants have been less than completely satisfactory. The organic and inorganic type coatings typically act as a passive, physical barrier against salt, moisture and the like without providing substantial corrosion protection. Cadmium plated fasteners and wet installation approaches, although finding considerable success in inhibiting corrosion of aluminum structures, have other undesirable limitations, such as an embrittling effect on titanium and high strength steel in direct contact with cadmium. Wet installation imposes undesirably high cost of assembly and presents production adaptability problems and the like.
Chromates have been widely used for over many years as corrosion inhibitors in corrosion inhibiting coatings such as paints, sealants and caulking compounds. Commonly used corrosion inhibitors in the aerospace industry included alkaline earth and zinc salts of hexavalent chromium, which can also enhance adhesive properties of corrosion inhibiting compositions. The general theory of chemical corrosion inhibition action in coatings containing strontium chromate is that the chromate undergoes an oxidation reaction in the presence of water and in between two materials that are dissimilar with respect to galvanic potential. This reaction will typically result in a buildup of an oxide layer on the surface of aluminum with which a fastener made of a corrosion resistant metal alloy, such as titanium for example, is in contact. This oxide layer passively resists the propagation of galvanic corrosion between the interactive materials. However, it is now also generally accepted that such chromates can be toxic, and that the continued usage of chromates in corrosion inhibiting coatings represent health and environment hazards.
One passive corrosion resistant, protective metal-organic base coating that was developed many years ago for non-aluminum metal fasteners has been used in aluminum structures of aircraft, to counter the bimetallic corrosion that the non-aluminum metal fasteners would otherwise cause. That coating includes a mixture of a powdered metallic substance such as powdered aluminum or molybdenum disulfide with a phenol-formaldehyde resin in a volatile carrier selected from lower alkyl alcohols, such as ethyl alcohol, methyl ethyl ketone and petroleum distillate, such as toluene, together with strontium chromate and zinc chromate.
Another type of passive non-chromate, corrosion-inhibiting coating composition for metal surfaces includes at least one inhibitor selected from the group consisting of phosphates, phosphosilicates, silicates, and mixtures thereof, with at least one inhibitor being selected from titanates and zinc salts. The composition may also include a borate such as boric acid, and a succinate. A preferred phosphate includes calcium dihydrogen phosphate, and a preferred titanate is sodium titanium oxide. The zinc salt may include zinc phosphate or zinc cyanamide.
Another passive coating composition is also known that contains about 8% by weight of a salt of inorganic constituents including cations of zinc and calcium, and anions silicates, phosphates, carbonates and oxides, and about 8% by weight of 1-(Benzothiazol-2-ylthio) succinic acid, (BTTSA), suspended in a phenol-formaldehyde thermosetting resin. The remainder may further include a pigment such as molybdenum disulfide, aluminum, polypropylene, or combinations thereof. The corrosion resistant composition typically is dissolved and applied in a volatile solvent carrier. A variation of this coating material contains about 4% by weight of a salt of inorganic constituents including cations of zinc and calcium, and anions of silicates, phosphates, carbonates and oxides, about 4% by weight of 1-(Benzothiazol-2-ylthio) succinic acid, (BTTSA), and approximately 4% by weight of a BTTSA amine complex, suspended in a phenol-formaldehyde thermosetting resin. However, the shelf-life of this chromate free coating composition is approximately three to six months depending on storage temperature. As a result of this short shelf-life, small lot sizes of the coating must be utilized to ensure that inventory is fully used before that shelf-life expiration. Furthermore, it is necessary to make lots exclusively in response to customer orders to maximize the storage life of the material. These requirements increase production costs and decrease logistical flexibility. It has been found that this short shelf-life is due to the introduction of BTTSA as an acidic corrosion inhibitor, which triggers a phenolic resin cross-linking reaction. As a result of this cross-linking, a gelatinous yellow precipitate of phenolic resin forms at the bottom of containers of the coating composition. In addition, the application of these types of BTTSA containing coating compositions on fastener systems, for example by spray or dipping, results in an undesirable tackiness between fasteners, resulting in a degradation of the coating on the parts due to the creation of areas lacking the coating at contact points between the coated parts.
Galling in the form of surface damage of mechanically locked internally threaded fasteners used in the aerospace industry is also a common problem. Such galling typically arises between sliding solid parts, distinguished by macroscopic, usually localized, roughening and creation of protrusions above the original surface, and often includes plastic flow or material transfer or both. There remains therefore a need in the aerospace industry for a wear resistant coating that can result in a reduction of galling of mechanically locked internally threaded fasteners used in multiple torque cycling applications.
It also be would be desirable to provide an anti-corrosion coating to provide a barrier at a junction between fastener parts and an aluminum member with which it is attached that actively resists permeation of water molecules, in addition to passive corrosion resistance, to further limit corrosive effects that galvanic corrosion can have on fastener parts. A need thus still exists for coating formulations that do not contain chromates, but that combine useful passive and active corrosion resistance properties of different corrosion inhibitors to synergistically achieve an effectiveness substantially equivalent to that of chromate containing coatings in preventing corrosion, and reduction of galling of threaded fasteners. The present invention meets these and other needs.